1. Grammar check: For a formula, its grammar must conform to logical or mathematical rules. For example, in propositional logic, combinatorial formulas must conform to the rules for the use of logical symbols, such as the pairing of brackets and the correct use of propositional symbols. If there are grammatical errors in the formula, it is not a compound formula.

2. Semantic check: compound formulas must conform to specific semantic rules. For example, predicate logic, a formula must conform to the rules of using quantitative symbols and the correct use of predicate symbols, otherwise it is not a compound formula.